Mobility and clustering of Rh and Ru in zeolite. Experiment and theory

Ruthenium and rhodium have been atomically dispersed on Na zeolite as shown by the infrared spectra of Rh-I(CO)(2) and Ru-I(CO)(2) produced upon reduc tion in a CO atmosphere. Formation of adsorbed CO bridging two Rh(I) surfac e atoms upon partial removal of adsorbed CO indicates some Rh(I) species ca n occupy adjacent surface sites contrary to suppositions in most current li terature. While Rh(I) surface species seem to be relatively immobile below 100 degrees C, our data suggest that Rh-0 surface atoms are mobile to some extent. Self-consistent field density functional calculations have been per formed for several surface clusters; they indicate that Rh-surface bonds ar e much weaker for Rh-o species than for Rh(I) species. These calculations s upport the proposition that Rh-0 surface species are more mobile than Rh(I) surface species.